1. Field of the Invention
The present invention relates to a clutter immune radar.
2. Description of the Prior Art
In conventional radar detecting and ranging systems, an interrogation signal is transmitted toward a target and a skin-reflection is returned from the reflective surfaces of the target back to the interrogating unit. A problem, however, arises with such conventional systems in that skin-reflections from targets in critical relative positions are not discriminated from reflections from extraneous nonthreatening or "don't care" targets. Such skin reflectons from extraneous targets, are termed "clutter returns." Clutter returns are undesirable in that clutter can mask out returns from critical targets, or cause false alarms. For example, in the case of automotive collision avoidance radars, clutter returns can cause inopportune deployment of automatic braking or passenger resraints such as air bags.
Cooperative radar detecting and ranging systems are responsive only to specifically tagged targets which are capable of generating a reply signal that is in some way distinguishable or discriminated from the radar interrogaton signal and from skin reflections. Such cooperative radar systems, accordingly, are not subject to clutter. Such a cooperative system is described in U.S. Pat. No. Re.28,302 to H. Staras and J. Shefer dated Jan. 14, 1975. Another radar system, relatively immune to clutter, is descirbed in U.S. Pat. No. 4,003,049 entitled "Dual Mode Automobile collision Avoidance Radar" issued Jan. 11, 1977, to F. Sterzer G. S. Kaplan.
In some applications, it is desirable to incorporate in a single cooperative system, a ranging system and a communication system. For example, in navigation systems, it is desirable to range upon an identifiable reference buoy to determine thereby the exact absolute position of a vessel. It is noted, however, that provisions must be made to ensure that the communication signaling does not substantially interfere with the ranging process.
A combined ranging system and communications link for automotive vehicles is described in U.S. Pat. No. 4,001,822 entitled "An electronic License Plate for Motor Vehicles, " issued Jan. 4, 1977, to F. Sterzer. To ensure that the communication signaling does not substantially interfere with the ranging process, the Sterzer system utilizes a particular message format, including "a no-code" field wherein no additional modulation is imparted to the reply signal.
It is desirable to have a system wherein the communication signaling also acts as a discriminant, to thus aid, as opposed to merely not interfere with, the ranging process.